This invention is related to a method of operation of a nuclear power plant, and particularly to a method suitable for operation of a boiling water reactor power plant.
Generally, corrosion products, such as metallic ion components and insoluble components (clad) and the like, are dissolved little by little into water from structural materials, such as piping, pumps, heat exchangers, etc., of a boiling water reactor power plant (hereinafter referred to as BWR power plant). Most of the corrosion products formed in a turbine system on an upper side of a clean-up system for condensate water are removed by the clean-up system. But, the corrosion products formed in a feed water system at a downstream side of the clean-up system for condensate water are not removed. These corrosion products enter into the nuclear reactor together with the feed water. Most of the corrosion products which enter into the nuclear reactor deposit on the surface of the fuel rods. The corrosion products which are deposited on the surface of the fuel rods are transformed into radioactive nuclides by irradiation with neutrons. For example, Ni and Co contained in the corrosion products are transformed into .sup.58 Co and .sup.60 Co, which have a long half life, respectively, by neutron irradiation. A part of the corrosion products, being a radioactive substance, dissolves from the surface of the fuel rods into the reactor water, or exfoliates from the surfaces of the fuel rods. The dissolved or the exfoliated corrosion products deposit on the inner surfaces of equipment and pipings of a primary loop recirculation system or the clean-up system for the reactor water. Therefore, the dosage rate of the equipment and the pipings of the primary loop recirculation system, or of the reactor water cleanup system, increases. In the case where NiFe.sub.2 O.sub.4 and CoFe.sub.2 O.sub.4 are composite oxides of the corrosion products of Ni and Co that deposit on the surfaces of the fuel rods, the speed of re-dissolution of the composite oxides into the reactor water, or the speed of their exfoliation from the surfaces of the fuel rods, becomes very slow. The composite oxides stay on the surfaces of the fuel rods for a long time after they have deposited. As a result, re-deposition of the corrosion products, which are radioactive substances, on the equipment and pipings is suppressed. Ni and Co in the reactor water form stable composite oxides by setting a weight ratio of Fe/Ni in the feed water to 2 or more (actually, about 3). In order to obtain the Fe/Ni weight ratio, a technique for controlling the iron concentration in the feed water has already been adopted in the BWR. However, even when the Fe concentration is controlled, as mentioned above, a new phenomenon was observed wherein the concentration of the radioactive substances, such as .sup.60 Co, in the reactor water changes. The reason is that the concentration of the radioactive substances in the reactor water does not stabilize due to an increase of the quantity of Cr dissolving from stainless steel pipes used for heat transfer pipes of the feed water heater. The increase in the dissolution quantity of Cr accelerates re-dissolution of clad deposited on the fuel rods. Several measures are described in Japanese patent Laid-open Print No. 5-288893. These measures suppress a fluctuation in the concentration of radioactive substances in the reactor water, to control the Cr quantity in the deposit on the fuel rods in addition to controlling the Fe quantity. The Laid-open Print describes a method for reducing the Cr content in clad deposited on the fuel rods by methods such as a method for properly managing the concentration of an oxidizing agent in the feed water, a method for substituting a material having a high Cr content for a low Cr content material, a method for making alkali ions coexistent with Cr ions in the reactor water, and a method for efficiently operating the clean-up equipment for cleaning the reactor water. Japanese Patent Publication No. 68914 describes a technique that controls the pH of the reactor water to a weak alkali state. This is the technique that suppresses an uptake of radiation to the main pipe and is applied only at the time of the starting operation of the nuclear reactor when the uptake speed is fast. But, Japanese Patent Laid-open Print No. 5-288893 does not pay attention to the injection time of an alkali metal. The technique disclosed in Japanese Patent Publication No. 6-8914 is applied only to controlling the weak alkali state at the time of starting, but this technique cannot suppress Cr loading on the fuel rods in new fuel assemblies to be loaded during every operating cycle.